1. Field of the Invention
The present invention relates to optical elements, optical pickup units, and optical disk drive units, and more particularly to an optical pickup unit employed in an optical disk drive unit that can record information on and/or reproduce information from optical recording media of a plurality of types such as a compact disk (CD) type and a digital versatile disk (DVD) type, an optical element employed in such an optical pickup unit, and an optical disk drive unit employing such an optical pickup unit.
2. Description of the Related Art
Conventional optical disk drive units that can record information on and/or reproduce information from both DVD-type optical disks such as DVDs and S-DVDs and CD-type optical disks such as CDs, CD-Rs, and CD-RWs require two light sources of different wavelengths. That is, a light source of a red region of 630 to 680 nm is used for the DVD-type optical disks and a light source of a near infrared (IR) region of 770 to 800 nm is used for the CD-type optical disks. Information can be recorded on or reproduced from the CD-type media excluding the CD-Rs by a red light source. The CD-Rs require a near IR light source for information recording or reproduction since the CD-Rs, which employ a pigment compound for their recording layers, have a narrow absorption bandwidth, so that information cannot be recorded thereon or reproduced therefrom by the red light source.
In the case of requiring two light sources as described above, the simplest way is to mount two optical pickup units separately for DVDs and CDs in an optical disk drive unit. At this point, by setting the wavelength λ of the light source of the CD optical pickup unit to 785 nm, the CD-Rs may be used for information recording or reproduction. However, it is difficult to achieve downsizing and cost reduction of the optical disk drive unit by this method employing the two optical pickup units.
Accordingly, two optical systems are provided in the housing of one optical pickup unit. FIG. 1 is a diagram showing a conventional optical pickup unit including two optical systems.
In FIG. 1, a linearly polarized divergent light emitted from a semiconductor laser (LD) 101 of a wavelength of 660 nm for the DVD optical system is formed into a substantially parallel light by a collimator lens (CL) 102 to travel through a polarization beam splitter (PBS) 103. Further, the light passes through a ¼ wave plate (phase plate) 105 for 660 nm to be circularly polarized in a first direction. Thereafter, the light passes through a dichroic prism (dichroic element) 104 and has its light path (emission path) deflected by a deflection prism (DP) 106 by 90°. Then, the light is incident on an objective lens (OL) 107 (a light-converging part) to be converged into a small spot on an optical recording medium 108. Information reproduction, recording, or erasure is performed by the spot. The light reflected back from the optical recording medium 108 is circularly polarized in a second direction reverse to the first direction to be formed again into a substantially parallel light by the OL 107. The light is deflected by the DP 106 and passes through the dichroic prism 104. Then, the light passes through the ¼ wave plate 105 and is linearly polarized to be perpendicular to its emission path. Thereafter, the light is reflected from the PBS 103, formed into a convergent light by a condenser lens (DL) 109, and reaches a light-receiving element (PD) 110. An information signal and servo signals including a tracking-servo signal and a focus-servo signal are detected from the light-receiving element 110.
Next, a description will be given of the CD optical system. In recent optical pickup units for CDs, a hologram unit (HOE unit), which is formed by providing light-emitting and light-receiving elements in one container (can) and separates a bundle of rays by using a hologram (HOE), has been used commonly. In the optical pickup unit of FIG. 1, an HOE unit 201, which is formed by providing a semiconductor laser chip (LD chip) 2011 and a light-receiving element (PD) 2013 in one can to separate a bundle of rays by using a hologram (HOE) 2012, is also provided for the CD optical system.
In FIG. 1, a divergent light of 780 nm emitted from the LD chip 2011 of the HOE unit 201 is coupled by a coupling lens 202 and reflected from the dichroic prism 104. Then, the light has its light path deflected by the DP 106 by 90° and is incident on the OL 107 to be converged into a small spot on the optical recording medium 108. Information recording, reproduction, or erasure is performed by the spot. The light reflected back from the optical recording medium 108 is formed again into a substantially parallel light by the OL 107. The light is deflected by the DP 106 and reflected from the dichroic prism 104. Then, the light is formed into a convergent light by the coupling lens 202, diffracted by the HOE 2012 toward the PD 2013 provided in the same can as the LD chip 2011, and received by the PD 2013. An information signal and servo signals including a tracking-servo signal and a focus-servo signal are detected from the PD 2013. FIG. 2 is an enlarged fragmentary schematic view of the HOE unit 201.
Japanese Laid-Open Patent Application No. 6-295464 discloses an optical pickup unit employing a ¼ wave plate, which serves to improve usability of light in the optical pickup unit. The ¼ wave plate converts a linearly polarized light emitted from a laser light source to a circularly polarized light so that the circularly polarized light is projected on a disk. Further, the ¼ wave plate converts the circularly polarized light reflected from the disk to a light that is linearly polarized in a direction perpendicular to the linearly polarized light emitted from the laser light source and leads the linearly polarized light to a light-receiving element without loss of light. The ¼ wave plate can improve usability of light. However, the light circularly polarized by the ¼ wave plate is prone to have a phase difference on a reflection surface. Therefore, it is desirable that the ¼ wave plate be provided on a light path in the vicinity of an objective lens.
For instance, it is desirable in terms of phase difference management to provide the ¼ wave plate right beneath the objective lens. However, providing the ¼ wave plate right beneath the objective lens causes the problem of an increase in the thickness of an optical drive unit including the optical pickup unit in a vertical direction. Therefore, in order to reduce the thickness of the optical drive unit, the ¼ wave plate may be omitted by providing an upward reflection mirror also provided under the objective lens with the function of the ¼ wave plate.
Recently, there have been developed optical disk drive units called multiwriters that can perform recording and reproduction with respect to media of both CDs and DVDs. In order to enable both CD and DVD recording, a high usability of light is required for each of the two types of media. Consequently, a ¼ wave plate is required that functions as a ¼ wave plate for both lights of a wavelength of 780 nm employed for the CDs and a wavelength of 650 nm employed for the DVDS.
However, a normal wave plate cannot be provided with the function of a perfect ¼ wave plate for both wavelengths. Therefore, the normal wave plate is prevented from providing a phase difference of 90° (a ¼ wavelength) between the incident lights of both wavelengths, but can only provide a slightly increased or decreased phase difference such as 100° or 80°. In such a case, usability of light diminishes for a deviation from 90°.